Due to increasing demands for improvement of fuel efficiency and enhanced regulation of exhaust gas, an environmentally-friendly vehicle, such as a hybrid electric vehicle, has been developed.
A general hybrid electric vehicle combines an engine and a motor as driving sources, and uses fossil fuel energy and electrical energy for travelling, thus decreasing exhaust gas and improving fuel efficiency.
FIG. 1 schematically illustrates a power train of a transmission mounted electric device in which a driving motor 3 and a transmission 4 are mounted in a hybrid electric vehicle.
As illustrated in FIG. 1, an engine 1 and the driving motor 3, which serve as driving sources for driving the vehicle, are serially connected. The transmission 4 is connected to an output side of the driving motor 3 to shift power of the engine 1 and the driving motor 3 and to transmit the shifted power to a driving shaft of the hybrid electric vehicle.
An integrated starter and generator (ISG) 5, which starts the engine 1 or performs power generation with the power transmitted from the engine, is connected to the engine 1 to transfer the power.
A battery (not illustrated), which serves as a power source of the driving motor 3 and the ISG 5, is chargeable/dischargeable connected to the driving motor 3 and the ISG 5 through an inverter (not illustrated), and the inverter converts a direct current of the battery into a three-phase alternating current and applies the three-phase alternating current for driving the driving motor 3 and the ISG 5.
An engine clutch 2 is disposed between the engine 1 and the driving motor 3 and selectively blocks power between the engine 1 and the driving motor 3.
The engine clutch 2 selectively connects or blocks the power between the engine 1 and the driving motor 3 through an engagement (lockup) operation and a release (open) operation by an actuator (not shown).
The engine clutch 2 may be divided into a wet clutch and a dry clutch, and the actuator is controlled according to a control signal of an engine control unit (ECU), so that the engine clutch 2 is engaged or released.
In the hybrid electric vehicle as described above, a travel mode is selected according to an operation condition, and the hybrid electric vehicle travels in an electric vehicle (EV) mode that is a pure electric vehicle mode using only power of the driving motor 3 or in a hybrid electric vehicle (HEV) mode using power of the engine 1 and power of the driving motor 3 together.
When the vehicle brakes or coasts by inertia, a regenerative mode for collecting brake and inertia energy through power generation of the driving motor 3 and charging the battery is performed.
The ISG 5 is operated as a power generator by the power of the engine 1 itself or a power generator by a rotary force transmitted through the engine under a regenerative condition to charge the battery.
In the hybrid electric vehicle, the engine clutch is engaged or released according to acceleration/deceleration by driver's manipulation of an acceleration pedal and a brake pedal, loads, a vehicle speed, a state of charge (SOC) of the battery, and the like, so that the hybrid electric vehicle travels in the HEV mode or the EV mode.
When the EV mode is shifted to the HEV mode while driving, the engine clutch is engaged after an engine speed is synchronized with a speed of the driving motor, so that a torque does not vary during a power transmission process between the engine and the driving motor, thereby securing operability.
More particularly, when the EV mode is shifted to the HEV mode, when a difference between speeds of each terminal of the engine clutch, that is, a speed of the engine and a speed of the driving motor, enters a predetermined speed difference by controlling the speeds of each terminal of the engine clutch after cranking of the engine by the ISG, the engine clutch slips.
Next, when a predetermined time elapses after the slip control of the engine clutch starts, it is determined that the speed of the engine and the speed of the driving motor are synchronized and the engine clutch is completely engaged, thereby completing the shift from the EV mode to the HEV mode.
A position (torque transmission start point) at which friction surfaces (clutch surfaces) of both ends of the engine clutch are in contact with each other and the torque transmission start point, that is, a position of a contact point at which the engine clutch starts to be in a slip state, is referred to as a touch point (or a kiss point (KP)) of the engine clutch.
In transmitting power of the engine to a wheel (motor) through the clutch slip by the general engine clutch, it is important to accurately recognize the touch point that is the start position of the transmission of power of the engine through the clutch and to accurately detect, by the ECU, a physical transmission quantity of the transmission torque of the engine clutch.
A general hybrid system introduces engine clutch learning control, and engine clutch learning may generally be divided into touch point learning and engine clutch transmission torque learning.
The touch point learning is learning of searching for a position of an actuator (a stroke position of a piston to be described below) at a start time point of the power transmission in the dry clutch, and when the touch point is incorrectly learned, an undesired transmission torque is generated, so that a difference sense may be generated. When an offset is large, a control response property of the clutch may deteriorate.
Engine clutch performance may vary according to progress of abrasion of the contacted friction surfaces. When abrasion of the engine clutch progresses, a clamping load and a release load are increased, and the touch point of the clutch is also changed according to an abrasion state.
Accordingly, when the touch point is not learned, a force may be applied on the friction surfaces before the clutch reaches an actual touch point, and an undesired clutch transmission torque and a difference sense may be generated.
When a difference between a learning value and an actual touch point is large, a clutch control response property may deteriorate, such that it is necessary to accurately and continuously learn the touch point in order to accurately control a transmission torque of the clutch.
Particularly, the dry clutch has a larger abrasion degree and higher importance of learning of the touch point than the wet clutch, and in order to properly learn the touch point, as illustrated in FIG. 2, it is necessary to decrease an influence of a temperature using a fluid.
In an actuator which operates an engine clutch by using a working fluid as illustrated in FIG. 2, when a motor 12 is driven according to a command of an engine control unit 11, a rotary force of a screw shaft 13 is converted into a straight force and transmitted to a piston 14, and a pressure of the working fluid supplied to the engine clutch 2 is controlled while the piston 14 moves forward or backward to control the piston 14.
A travel sensor 16 for detecting a stroke position of the piston 14 and a pressure sensor 17 for detecting pressure of a working fluid are installed at a master cylinder 15.
When learning a touch point, the engine control unit 11 determines a start time point of a torque transmission of the engine clutch 2, and then stores a value of the stroke position of the piston 14 detected by the travel sensor 16 at the start time point in a memory as a touch point value (final learning value).
A system of FIG. 2 includes a slave cylinder 19 receiving pressure of the working fluid from the master cylinder 15 and applying a force to the engine clutch 2, and a fluid pipe 18 is connected between the master cylinder 15 and the slave cylinder 19 so that the fluid pipe 18 is filled with the working fluid to transmit a pressure of the working fluid.
As described above, the force for operating the engine clutch 2 is transmitted through the fluid filled inside the fluid pipe 18, so that in order to increase accuracy of a touch point learning value, it is necessary to remove a temperature influence on the fluid.
Accordingly, a refill control for fully filling the fluid pipe 18 with a fluid may be performed before the touch point learning of the engine clutch is performed so that a temperature influence may be removed.
When the piston 14 of the master cylinder 15 completely pushes the fluid, the fluid pipe may be fully filled with the fluid, so that the refill control may be performed in a state where the engine clutch 2 is completely engaged.
That is, the refill control for fully filling the fluid pipe 18 with the fluid in a state where the pistons 14 and 20 of the actuator move a stroke so that the engine clutch 2 is completely engaged may be performed.
FIG. 3 is a diagram illustrating a comparison between touch point values before and after a refill control, and A represents a stroke position value of the piston 14, and {circle around (1)} represents a piston stroke position value at the time of learning of a touch point at which the engine clutch 2 is slightly engaged and then released until a contact between friction surfaces starts (until a torque transmission starts) in order to calculate a touch point value, and {circle around (2)} represents a piston stroke positon value when the engine clutch 2 is completely engaged and a refill control for filling the fluid pipe 18 with the fluid is performed.
A-1 represents a piston stroke positon value at the time of the learning for obtaining a touch point value in a state where the refill control is not performed, and A-2 represents a piston stroke positon value at the time of the learning for obtaining a touch point value when the engine clutch 2 is completely engaged and the refill control is performed.
B represents a touch point value, B-1 represents a touch point value obtained in the state where the refill control is not performed, and B-2 represents a touch point value obtained in the state where the engine clutch is engaged and the refill control is performed.
As described above, in comparison between the case where the refill control is performed and the case where the refill control is not performed, a difference in the touch point value is large, and a difference in a transmission torque is several tens of Nm, so that the difference in the transmission torque greatly influences operability.
Accordingly, in order to accurately obtain a touch point value and improve accuracy of the touch point learning, the engagement of the engine clutch and the refill control indicated by {circle around (2)} may be performed before the learning of the touch point indicated by {circle around (1)} is executed.
However, in the related art, the touch point is learned at a P position of a gear shift stage that is before the vehicle starts, and subsequently the refill control is performed when the engine clutch 2 is engaged after a vehicle starts running, so that the fluid pipe 18 may not be filled with the fluid at the time of the learning of the touch point before the vehicle starts.
Particularly, when the refill control is performed while the vehicle travels, the vehicle is operated again long after the operation of the vehicle is ended, and the touch point learning process is performed before the vehicle starts, it cannot be determined whether the fluid pipe is refilled with the fluid at the time of the learning. A fluid state within the fluid pipe at the time of a re-start of the operation after a long time has passed may be similar to a state where the refill is not performed.
Accordingly, the touch point value obtained after the learning and the transmission torque of the engine clutch after the learning exhibits large differences similar to the difference between the case where the refill is performed and the case where the refill is not performed as illustrated in FIG. 3.
Accordingly, there exists for an improved touch point learning method.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.